1. Field of the Invention
This invention relates to a paramagnetic oxygen analyzer for detecting the concentration of oxygen gas contained in a sample gas, through utilization of the magnetic properties of the oxygen gas.
2. Discussion of the Prior Art
Paramagnetic oxygen analyzers are classified as a gas analyzer which measures an oxygen concentration in a sample gas by utilizing the property of oxygen gas that has a very large susceptibility as compared to the other gases in the sample gas. Depending on the principle of measurement used, there are three known types, (1) the thermomagnetic type, (2) the magnetic susceptibility type, and (3) the magnetic pressure type.
The thermomagnetic type is shown in FIG. 1 and comprises a measuring cell 1, a closed loop-like measuring passage 101 forming a measuring chamber, an inlet 102 and an outlet 103 for a sample gas Sg disposed at symmetrical positions along the measuring passage 101, and a bypass pipe 104 connected to two circular measuring passage segments which extend between inlet 102 and outlet 103. A heating resistance wire R.sub.1 is wound around bypass pipe 104, and a magnetic field Mf is generated in one of the connecting portions between bypass pipe 104 and measuring passage 101. With resistors R.sub.1, R.sub.2 and R.sub.3 forming a bridge, a detecting circuit is adapted to measure an unbalanced voltage appearing between the midpoint of R.sub.1 and the connecting point of R.sub.2 and R.sub.3.
The operation of the foregoing analyzer is as follows. Where oxygen gas is included in the sample gas, oxygen molecules are attracted to magnetic field Mf to flow into bypass pipe 104. The oxygen molecules having flowed in, are heated by heating resistance wire R.sub.1. When heated, the susceptibility of the oxygen molecules decreases, so that the force with which the oxygen molecules are attracted to magnetic field Mf weakens. Then, the oxygen molecules are pushed by new low-temperature oxygen molecules coming in, so that they flow through bypass pipe 104. The bridge circuit detects a temperature change due to such a magnetic wind. Thus, the oxygen concentration in the sample gas Sg can be measured.
However, in this type of analyzer, the sample gas introduced into bypass pipe 104 by the attractive action of the magnetic field Mf is heated, so that the heat conductivity, heat capacity, viscosity, etc, of the gas components, other than the oxygen gas included in the sample gas Sg, cause an adverse influence which result in an interference error.
The magnetic susceptibility type is shown in FIG. 2 and comprises diamagnetic dumbbells 105 and 106 suspended horizontally in a magnetic field Mf generated by pole pieces 107,108,109,110, and a sample gas Sg is caused to flow through such an area. Servomex markets a typical oxygen analyzer of this type. Oxygen molecules, included in the sample gas Sg, are attracted to the magnetic field Mf, so that diamagnetic dumbbells 105 and 106 are pushed out of magnetic field Mf. The positional displacement, which may be the torsion, of dumbbells 105 and 106 corresponds to the oxygen concentration. Thus, by detecting the positional displacement and supplying a feedback current, corresponding to the displacement, to a coil 111 wound around dumbbells 105,106, such that a counter torque produced by means of an electromagnetic effect returns dumbbells 105,106 to their initial state, it is possible to obtain the oxygen concentration from the feedback current.
However, disadvantageously, in this type of analyzer, because the pair of dumbbells is suspended horizontally in the magnetic field, its structure is complicated and susceptible to mechanical shock and vibration.
The magnetic pressure type is shown in FIG. 3. An example of such analyzer is disclosed in German Pat. No. 2701084. In FIG. 3, similar members as shown in FIG. 1 are designated by the same reference symbols and will not be described hereat for clarity of description. In addition, the analyzer comprises a connecting pipe 112 for connecting two measuring passage segments which extend between inlet 102 and outlet 103, a micro-differential pressure detector 113, such as a capacitor microphone, provided at the center of the pipe, and a purge gas passage 114 which allows a purge gas Pg, supplied from an inlet 115, to flow through throttles 116 and 117, into pipe segments 112 located on either side of detector 113. Magnetic fields Mf, Mf' are generated alternately and are in the two connecting portions between pipe 112 and measuring chamber 101.
In the FIG. 3 embodiment, oxygen molecules included in sample gas Sg are attracted to each magnetic field Mf,Mf', so that the background pressure of an area, where the magnetic field is generated, increases. Because magnetic fields Mf and Mf' are generated alternately, an alternating signal whose amplitude corresponds to the oxygen concentration, is produced by detector 113.
In this type of analyzer, although there is only a small amount of interference error, detection is susceptible to mechanical vibration and shock being applied to the detection portion because only one detector is used. Furthermore, because the detector 113 is configured so as to detect a micro pressure transmitted to the detecting portion, it is susceptible to background pressure change. Thus, the device has the disadvantage that a measurement error will occur when a pressure change on the downstream side of outlet 103 is fed back.
Thus, there is still a need in the art for a paramagnetic oxygen analyzer that is substantially error free.